Flexographic printing is widely used in the production of newspapers and in the decorative printing of packaging media. In flexographic printing, a layer of a flexible printing medium is deposited onto a flexible substrate such as a thin sheet of steel, aluminum, or synthetic polymer, to form a printing plate or a printing sleeve. A relief pattern corresponding to the negative image to be printed is formed in the printing medium. The plate is then mounted on the printing press, and printing commences.
In the manufacture of flexographic printing plates, photosensitive printing material is coated onto the substrate to form the printing plate or printing sleeve. The coated side is exposed with light to form a negative of the image to be printed, causing photopolymerization of the exposed portion of the printing medium, which then becomes physically hardened and resistant to solvent removal. The unexposed and therefore unhardened portion of the printing medium is removed by washing with solvent, leaving a relief pattern of the image to be printed. The printing plate is mounted on a press and printing commences.
Non-flexographic printing plates such as letterpress plates are also used for printing newspapers, shoppers, and books. Photosensitive resin compositions have been developed for use with non-flexographic printing applications for the same reasons disclosed above for flexographic applications. The use of photosensitive printing media for the manufacture of letterpress printing plates is essentially the same as for flexographic printing applications.
Direct cure refers to an imaging approach wherein the photopolymer of the plate (or other printing element) absorbs some of the actinic radiation causing a chemical reaction that polymerizes (i.e., cures) the photopolymer, rendering it insoluble in the washout solvent. Coherent energy, i.e., actinic radiation, is directed onto the surface of the photosensitive matrix in the desired pattern.
Various means have been proposed to increase the direct-cure imaging speed of flexographic and letterpress printing elements upon exposure to lasers and other digital sources of actinic radiation. For example, efforts have been made to develop more highly reactive photosensitive resins. Such materials would be expected to give more complete photoreaction (e.g., crosslinking, dissolution of crosslink bonds, rearrangement, and the like), even with brief laser exposures, as the desired image is scanned onto the photosensitive resin. One such reactive photosensitive resin system can be found in U.S. Pat. No. 5,976,763 to Roberts et al., the subject matter of which is herein incorporated by reference in its entirety.
Other efforts have focused on enhancing the imagewise exposure of a photosensitive material by using an apparatus that subjects photosensitive materials to a relatively low energy pre-exposure using the electromagnetic energy during the non-imaging portion of the exposure process (i.e., a backscan beam exposure) prior to subjecting the photosensitive materials to the main imaging exposure (i.e., an imagewise exposure). This concept is discussed in U.S. Pat. No. 6,262,825 to Mueller et al., the subject matter of which is herein incorporated by reference in its entirety. However, this process requires two exposure steps, thus increasing the time needed to process the photosensitive materials.
The inventors of the instant invention have found that the use of a highly reflective layer beneath the photosensitive resin layer can greatly enhance the imaging speed of photopolymer relief printing plates, while maintaining good resolution, when image-wise exposed using digital sources of actinic radiation. Instead of being absorbed by the reflective layer, the photons of actinic radiation are reflected back up into the photopolymer where they speed up the curing of the printing element.
While reflective layers have not previously been contemplated for use in flexographic or letterpress relief image printing elements, they have been suggested for use in other processes.
For example, highly reflective substrates have been proposed for use in producing image-receiving elements. U.S. Pat. No. 5,380,695 to Chiang et al., the subject matter of which is herein incorporated by reference in its entirety, disclose an image-receiving element comprising a support, wherein the support may comprise transparent, opaque or translucent material, with reflective (opaque) supports being preferred for the production of identification documents where image date is viewed against an opaque background. There is no suggestion in Chiang et al. that the reflective supports can be used in producing relief image printing plates.
Likewise, U.S. Pat. Nos. 5,468,540 and 5,670,096 to Lu, the subject matter of which is herein incorporated by reference in its entirety, describe a reflectroreflective article used as a transparent overlay to protect documents from tampering. Again, there is no suggestion that the reflective layer can be used to produce relief image printing plates.
U.S. Pat. No. 5,636,572 to Williams et al., the subject matter of which is herein incorporated by reference in its entirety, describes a surface layer below the IR-sensitive layer for reflecting IR radiation back into the IR-sensitive layer in order to increase net energy absorption and decrease laser power requirements. However, the invention described by Williams is directed to lithographic printing plates and is concerned with reflecting IR radiation back into the IR-sensitive layer, instead of the actinic radiation contemplated by the inventors of the present invention.
The inventors of the present invention have found that the benefit of high substrate reflectivity is particular to the imaging of plates with digital sources where the actinic radiation is substantially coherent, i.e., “high brightness.” Conventional exposure systems, which have non-coherent sources of actinic radiation, can actually be harmed by highly reflective substrates due to the substantial scatter of the reflected radiation into non-image areas. The harmful effect of high substrate reflectivity is discussed in U.S. Pat. No. 4,622,088 to Min, the subject matter of which is herein incorporated by reference in its entirety. Min describes that when highly reflective supports are used, oblique rays passing through clear areas in the image-bearing transparency will strike the surface of the base at an angle other than 90° and after reflection, will cause polymerization in the non-image areas. Min teaches that this disadvantage can be overcome when the photopolymer layer is on a radiation-reflective support by an intervening stratum sufficiently absorptive of actinic radiation so that less than 35% of the incident radiation is reflected. The layer absorptive of reflected radiation or nonradiation scatter layer or antihalation layer, can be made by dispersing a finely-divided dye or pigment which substantially absorbs actinic radiation in a solution or aqueous dispersion of a resin or polymer which is adherent to both the support and the photoinsolubilized image and coating it on the support to form an anchor layer which is dried. This concept is discussed also in U.S. Pat. No. 4,460,675 to Gruetzmacher et al. and U.S. Pat. No. 4,423,135 to Chen et al., the subject matter of which is herein incorporated by reference in its entirety.
Furthermore, U.S. Pat. No. 6,037,101 to Telser et al., the subject matter of which is herein incorporated by reference in its entirety discloses a photosensitive recording material wherein if highly reflective panels or sheets are used as the substrate, the reflective panels or sheets contain suitable antihalation agents, such as carbon black or manganese oxide. In the alternative, the antihalation agents can be applied as a separate layer to the substrate or may be present in the adhesion-promoting layer or in the photopolymer layer.
Thus, there is a clear need in the art for methods that will enhance the “imagewise” exposure sensitivity of photosensitive materials, thereby permitting photoimaging to proceed as rapidly as possible, allowing for the rapid conversion of the photosensitive materials into finished articles. Furthermore, there remains a need to for a digitally imageable flexographic relief printing element that can provide an increased direct-cure imaging speed upon exposure to coherent sources of actinic radiation.